1. Field of the Invention
The present invention relates to a preferably continuous process for preparing cyclododecatrienes (CDTs) over a catalyst system comprising, in particular, nickel or titanium, with removal of the crude cyclododecatrienes by distillation and recycling of the catalyst.
2. Description of the Background
The synthesis of CDT starting from 1,3-butadiene has been examined using both homogeneous and heterogeneous transition metal catalysts. In the case of heterogeneous catalyst systems, a transition metal complex is typically bound to a polymeric support via a bridging ligand (U. Schuchardt, J. Mol. Catal; 29 (1985) 145). Such fixed-bed systems have the serious disadvantage that the bridging ligand competes with butadiene, CDT and olefinic intermediates for a free coordination position on the catalyst. This competition considerably reduces the conversion rate of the fixed-bed catalyst, and the proportion of dimeric and/or oligomeric by-products generally increases. In addition, CDT can displace the bridging ligand from the transition metal. This displacement results in the metal atom being leeched from the fixed bed. The fixed-bed catalyst loses its active centers and its catalytic activity decreases.
Because of these problems, homogeneous catalysts rather than fixed-bed catalysts have generally become established in industrial-scale implementation of the synthesis. The advantages of homogeneous catalysts are, in particular, very good space-time yields and high selectivities in favor of CDT. Among the transition metals known in the literature (G. Wilke, Angew. Chemie 69 (1957) 397; H. Breil, P. Heimbach, M. Krxc3x6ner, H. Mxc3xciller, G. Wilke, Makromolekulare Chemie 69 (1963) 18; G. Wilke, M. Kroner, Angew. Chemie 71 (1959) 574), titanium, chromium and nickel compounds are used most often. These transition metals are catalytically active in the form of organometallic complexes in which the central atom is in the oxidation state 0. These organometallic complexes are typically prepared from a transition metal salt and a reducing agent. The reducing agent used is generally an organometallic compound of the first through third group of the Periodic Table. For the titanium catalysts widely used in industry, a useful route has been found to be, in particular, the reaction of titanium tetrachloride or titanium acetylacetonate with an organoaluminum compound (U.S. Pat. No. 3,878,258; U.S. Pat. No. 3,655,795, both of E.I. du Pont de Nemours; DE 30 21 840 and DE 30 21 791, both of Chemische Werke Huels), although numerous other titanium salts and reducing agents have also been described as starting compounds, e.g. DE 19 46 062, Mitsubishi Petrochemical Co. and U.S. Pat. No. 3,644,548, Asahi Chemical Industry.
In the industrial synthesis of CDT using a homogeneous catalyst, the reaction is usually conducted in a continuous process using one or more stirred vessels. Part of the reaction mixture is discharged continuously from the reactors. In the work-up, unreacted starting material is recovered and returned together with fresh butadiene to the reaction process. Part of the catalyst is also discharged together with the reactor output. The concentration of catalyst in the reactor is, therefore, usually kept constant by continuous addition of fresh catalyst constituents.
Before work-up of the reactor output, the catalyst which has been discharged has to be decomposed. Various polar solvents are very suitable for this purpose. Apart from water, Ube Industries use, for example, an ammonium hydroxide solution (JP 05-070377, JP 06-254398, both of Ube Industries, cited according to CA 119:72275 and CA 121:303571). Various alcohols (JP 07-625439, JP 07-625396, both of the Agency of Industrial Sciences and Technology, Japan, cited according to CA 86:17321 and CA 86:17322) are also suitable for this purpose. In particular, use is made of methanol (JP 07-442496, Toyo Soda Co., cited according to CA 82:139521) and methanolic hydrochloric acid (DE 19 42 729, Mitsubishi Petrochemical Co.).
The decomposition of the catalyst can also be conducted by means of acetone (JP 04-301345 1, Toyo Rayon, cited according to CA 70:77450) or by using a suspension of calcium oxide in water (NL 6 603 264, Shell Int. Research Maetschappij N.V.). Ube Industries comment that the CDT formed can be recovered only incompletely if water is used for decomposing the catalyst. However, the CDT yield can be improved if aqueous tetrahydrofuran is used (JP 05-070377, cited according to CA 119:72275).
All the above-mentioned examples of the homogeneously catalyzed CDT synthesis accept decomposition of the catalyst system during the work-up. Because of the high conversion rate and selectivity of the catalyst, the amounts of catalyst required compared to the amount of CDT formed are small, but an alternative work-up with recycling of the active catalyst would be desirable. This is particularly true of the two transition metals chromium and nickel because of the heavy metal contamination of the wastewater resulting from the work-up.
The industrial cyclodimerization of 1,3-butadiene to cyclooctadiene (COD) is conducted in the liquid phase over a nickel(0) complex. The homogeneous catalyst in this reaction comprises the transition metal and a bulky donor ligand, typically a phosphine or a phosphite. It is known that this catalyst system can be partly recovered and, therefore, used a number of times. For this purpose, the material discharged from the reactor is usually worked-up by fractional distillation. In this distillation step unreacted starting material, COD and low-boiling byproducts are separated. A relatively high-boiling residue remains in which the catalyst is present in dissolved form. This residue is returned to the reaction and the catalyst is used again for butadiene dimerization. Only after a number of cycles does the proportion of high boilers in the reaction mixture increase to such an extent that the catalyst has to be discharged with the high-boiling fraction and then must be discarded.
In the case of CDT (1,5,9-cyclododecatriene), which is formed as trans-trans-trans, cis-trans-trans and cis-cis-trans isomers, recycling of the catalyst on the industrial scale has not yet been described. Only a few examples, which were conducted batchwise and in which multiple use of the catalyst was attempted on a laboratory scale, are known. Thus, DE 30 21 791 A1 describes a process in which the catalyst is first adsorbed on activated carbon and is later removed by filtration together with the activated carbon. However, this process has not been found to be useful in industry, since the catalyst can be only partly recovered in this manner.
Example 3 of DE 12 83 836 describes a procedure in which the Ni(0)-COD complex after the reaction in the presence of the solvent benzene accumulates in the high-boiling bottom products of the distillation and still has a residual butadiene activity, so that it can be used once more upon recycling. However, no further information about catalyst recycling is given.
Example 7 of DE 28 25 341 shows that the catalyst after the reaction in the presence of the solvent and dibenzylbenzene moderator, which accumulates in the residue after distillation at 80xc2x0 C. and 0.5 torr and this residue, is reused in two further batch reactions. However, the catalyst displayed a significant loss in activity after use again in a third batch. A need, therefore, continues to exist for a process which enables the recycling of catalyst for effective reuse more than just a few times.
Accordingly, one object of the present invention is to provide a process for the synthesis of CDT in which the catalyst can be separated and recycled for use in the synthesis of CDT a plurality of times.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a process for preparing cyclododecatrienes from 1,3-butadiene in the presence of a catalyst system, which comprises:
reacting 1,3-butadiene in the presence of cyclooctadiene and/or cyclododecatriene in the absence of solvents extraneous to the system in the presence of a catalyst;
separating the crude cyclododecatriene product of the reaction by distillation from the catalyst; and
recycling from 50% to 100% of the catalyst system to a reaction medium for cyclododecatriene preparation.